Chemical-mechanical polishing (CMP) is performed in the processing of semiconductor wafers. A standard CMP apparatus has a circular polishing pad and a rotating carrier for holding a semiconductor wafer. An abrasive slurry is used on the polishing pad. After a CMP operation, residual particles are left on the surface of the semiconductor wafer. These residual particles need to be removed.
Semiconductor wafers are typically cleaned in a cleaning apparatus which includes one or more brush stations each having a pair of rotary brushes for cleaning the major surfaces of the wafers. A conventional cleaning apparatus 10 is illustrated in FIG. 1. The cleaning apparatus 10 includes a pair of brushes 12 that clean the major surfaces of a semiconductor wafer 14 placed therebetween. The semiconductor wafer 14 is supported by rollers 16, which are also used to rotate the wafer. The cleaning apparatus 10 includes at least one spray bar 18 to direct a spray of fluid towards the semiconductor wafer 14.
As semiconductor processes continue to achieve smaller line widths to create semiconductor wafers with greater capacity, post CMP defects control becomes more important for improving wafer yield and reliability. A key factor in brush cleaning is to precisely control the distance between the brush and the semiconductor wafer. If the separation distance is too tight, the residual particles from the CMP operation will scratch 20 the semiconductor wafer 14 as they are brushed off, as illustrated in a highlighted section 21 of the wafer as provided in FIG. 2. If the separation distance is to loose, some of the residual particles will not be removed. For example, a residual particle 22 may cause a short between two lines 24 and 26, as illustrated in a highlighted section 27 of the wafer as also provided in FIG. 2. Since the brushes 12 directly contact the semiconductor wafer 14 during cleaning, the separation distance is with respect to a reference position that is inward from a contact surface on the wafer.
Position of the brushes relative to the semiconductor wafer may be adjusted based on pressure sensors that detect pressure each brush applies to the wafer. For example, U.S. Pat. No. 5,475,889 discloses a brush assembly that includes a first rotary brush, a brush carriage having first and second arms, a second rotary brush, and a pressure adjustment assembly positioned to engage at least one of the arms of the brush carriage. The pressure adjustment assembly is configured to adjust the pressure applied to the wafer surfaces by the first and second rotary brushes. The brush assembly further includes a control system coupled to the pressure adjustment assembly for controlling operation of the pressure adjustment assembly to selectively increase and decrease the pressure applied to the wafer by the first and second rotary brushes.
As an alternative to monitoring pressure, the torque of a brush rotation motor may be monitored. U.S. Pat. No. 7,507,296 discloses monitoring the torque of a brush rotation motor while a brush is in contact with the semiconductor wafer and is being rotated by the motor. The position of the brush relative to the semiconductor wafer may be adjusted based on the monitored torque to regulate the pressure applied to the wafer by the brush.
Further developments on monitoring position of the brushes relative to the semiconductor wafer are still desired. This is particularly so as semiconductor processes continue to achieve smaller line widths to create semiconductor wafers with greater capacity.